1. Field of Invention
This invention relates to a structural window in a composite sandwich beam such as a ski or the wing of a composite airplane.
2. Prior Art
Modern skis are almost universally made as composite sandwich beams with a core of wood, foam or similar material and a skin. The skin contains structural materials with high tensile strength, a base and cosmetic sides and top. The structural materials for the skin could be aluminium, fiberglass cloth, carbon fiber cloth, or other material with a high tensile strength. A combination of these materials can also be used, e.g. aluminium and fiberglass together. The base material is often Ultra High Molecular Weight (UHMW) polyethylene. For the cosmetic sides and top ABS is often used.
The structural layers, base, and cosmetic layers are usually glued together with some form of epoxy, which makes a strong, flexible structure and a waterproof enclosure that protects the porous core from exposure to moisture.
The core is often protected from moisture at the tip and tail of the ski by replacing the core in these areas with a non-porous waterproof material such as aluminium or plastic.
The type and thickness profile of the ski core as well as the type and thickness of the skin layers affects how a ski flexes, which in turn is the primary determinant of how a ski performs and how strong the ski is. In addition, skin layers protect the ski core from moisture. If the core material absorbs moisture, the glue boundary between the core and the structural skin layer is compromised and catastrophic ski failure often results. Any discontinuities in the skin and/or core, such as holes or windows, can have serious detrimental effects on ski strength, flex, and performance.
In the past, the primary reason for making a hole in a ski through the cosmetic and structural layers was to mount ski bindings. Although screw holes are small enough to have minimal impact on the flex characteristics of the ski the problem that they pose for the moisture sensitive core is recognized. It is common practice to fill any unused holes in the ski as shown by Brichoud and Rullier U.S. Pat. No. 4,747,613 to ensure that the core always remains dry.
More recently vibration damping devices like that of Arieh and Courvoisier U.S. Pat. No. 4,563,020 and devices that detect the speed and displacement of the skier such as that of this inventor in U.S. Pat. Pub. No. 2004/0075737 or Schelling in German Pat. No. DE 100 14 253 or Gassner and Meier in German Pat. No. DE 195 24 842 have been introduced that require larger holes to be made in the ski and in some cases the holes need to penetrate the ski.
There is very little prior art related to a window in a composite sandwich beam, like a ski, that solves the problems related to moisture, strength, and flexibility. The closest related prior art include the hole in some mountaineering skis and the installation of bolts in composite airplane wings.
Most mountaineering skis, for example the Black Diamond “Virtige” or the Ski Trab “Aero 4” have a hole through the tip of the ski for securing the ski. This hole does not pass through the porous core of the ski, however, but through the non-porous waterproof material used at the tip. In addition, since the hole is in a part of the ski that does not affect the turning characteristics of the ski, nothing is done to maintain a constant flexibility in this area.
In “Understanding Aircraft Composite Construction” by Zeke Smith page 7-7 and 7-8 a method for attaching a bushing through a composite airplane wing is shown. In this method a bushing or insert is installed in the wing core and the structural outer skin is reinforced with additional structural outer skin layers. This method is not possible in a ski, however, where reinforcing the outer layers on the bottom of the ski would seriously affect the glide and flex characteristics of the ski. A different approach is required that redirects the tensile and compression loads in the outer skin through an internal structural element that allows the flex characteristics of the ski to be maintained without adding additional structural skin layers.
One of the reasons that no commercially available speed measurement devices have yet been introduced for skiers is because no one has successfully solved the problems related to installing a window in a ski that doesn't damage the ski structure, change the ski flex characteristics or expose the sensitive core to moisture.
The purpose of the present invention is to provide an apparatus and method for creating a hole through a ski that protects the core of the ski from moisture damage, has minimal effect on the strength and flexibility of the ski, allows the ski to be prepared (waxed, filed and ground) normally, and can be installed during the manufacturing of the ski or after the ski has been made.